An user called Seifer on XMage.de, after rage quitting, started joining my game.By continously requesting to watch my hand, blocked the game with thousands of windows requesting it.Even by closing the client and reopening it, the game would launch again, and thousands of windows would appear.

So you can just block any user by joining a game and clicking on request to see hand

Important things to fix, specially with morons like that seifer around.

EDIT: To prevent this go to preferencesdisable "Allow requests from players and spectators to show your hand cards". BOTH players must do this or the troll might still block your opponent.